1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic type of electrostatic recording type such as a copying machine, a laser beam printer, facsimile and the like.
2. Related Background Art
For example, various full-color image forming apparatuses having an image bearing member (photosensitive drum) of electrophotographic type for forming a toner image, and a transfer material bearing member for bearing a sheet (transfer material) onto which the toner image is transferred have been proposed and put to practical use. An example of such a full-color image forming apparatus is shown in FIG. 7.
In such a full-color image forming apparatus, around a photosensitive drum (image bearing member) 1 rotated in a direction shown by the arrow, there are arranged a corona charger 2, an exposure optical system 3, a developing means 4, a transfer device 5 and a cleaning device 6. Further, a cleaning device (transfer cleaner) 14 for cleaning a transfer drum 5a of the transfer device 5 is arranged around the transfer device 5.
The optical system 3 comprises a laser beam exposure device as shown having an original scanning portion and a color decomposing filter and adapted to illuminate a color-decomposed light image or a light image E corresponding to the color-decomposed light image onto the photosensitive drum 1. By successively illuminating the light images color-decomposed by the optical system 3 onto the photosensitive drum 1 negatively and uniformly charged by the charger 2, electrostatic latent images corresponding to different colors are successively formed on the photosensitive drum 1.
The developing means 4 is of rotatable type and comprises four developing devices, i.e., a yellow developing device 4Y, a magenta developing device 4M, a cyan developing device 4C and a black developing device 4K mounted around a rotary shaft 4b so that a desired developing device can be brought to a developing station opposed to the photosensitive drum 1, where the latent image formed on the photosensitive drum 1 is developed with toner having resin as a base component to form a toner image. The toner image formed on the photosensitive drum 1 is transferred onto a recording material supplied from a recording material supply cassette 7 to the transfer device 5 at a transfer station where the transfer device 5 is opposed to the photosensitive drum 1.
In this example, the transfer device 5 includes the transfer drum 5a as a transfer material bearing member which comprises a cylinder having a peripheral opening and a recording material (transfer material) bearing sheet 5f made of dielectric material and covering the peripheral opening. Around the transfer drum 5a, there are arranged a transfer charger 5b, an absorb charger 5c, an absorb roller 5g, inner and outer chargers 5d, 5e and a separation charger 5h. These chargers such as the transfer charger 5b and the like comprise corona chargers. The absorb roller 5g comprises a conductive roller.
The recording material supplied from the recording material supply cassette 7 to the transfer device 5 through a convey system is electrostatically absorbed on the bearing sheet 5f of the transfer drum 5a by means of the absorb charger 5c and the absorb roller (acting as a counter electrode) 5g so that the recording material can be conveyed toward the transfer station as the transfer drum 5a is rotated. At the transfer station, by applying an electric field to the recording material by means of the transfer charger (corona charger) 5b disposed within the transfer drum 5a, the toner image formed on the photosensitive drum 1 is transferred onto the recording material. The recording material to which the toner image was transferred is conveyed to the transfer station again by the rotation of the transfer drum 5a, where the toner image for a next color formed on the photosensitive drum 1 is transferred onto the same recording material.
After four color toner images are transferred onto the recording material in this way, the electricity is removed from the recording material by the separation charger 5h disposed outside the transfer drum 5a, and then, the recording material is separated from the bearing sheet 5f by a separation means 8. The separated recording material is sent to a fixing device 9, where the toner images are fused and mixed to be fixed to the recording material, thereby forming a permanent full-color image. On the other hand, the residual toner remaining on the photosensitive drum 1 and the bearing sheet 5f is removed by the cleaning device 6 and the cleaning device 14, respectively, for preparation for the next image formation.
Incidentally, convey paths for the recording material supplied from the cassette 7 and a recording material supplied by the manual insertion are shown by the broken lines in FIG. 7.
However, in the above-mentioned conventional image forming apparatus, the cleaning of the bearing sheet 5f is started during the separation of the recording material from the transfer drum 5a, and the cleaning is also started from a point (on the transfer drum 5a) where a tip end of the recording material was positioned. Thus, as shown in FIG. 8, if a distance between the transfer station (where the transfer drum 5a is opposed to the photosensitive drum 1) and the transfer cleaner 14 L2 along a rotational direction of the transfer drum 5a having the bearing sheet 5f is smaller than a peripheral distance (recording material peripheral length) L1 on a peripheral surface of the bearing sheet 5f of the transfer drum 5a, when the transfer cleaner 14 is driven as the point where the tip end of the recording material was positioned reaches the transfer cleaner 14, since the last color toner image is still being transferred onto the recording material, the color deviation of the last color toner image will occur due to the shock.
In order to eliminate such inconvenience, in the past, when the recording material peripheral length L1 is long, after the recording material is separated from the transfer drum, the transfer drum 5a is idly rotated by one revolution, thereby cleaning the transfer drum. In this case, however, in a both-face image formation (both-face copy) mode, it is impossible to prevent the deterioration of the image due to the oil contamination on the photosensitive drum. That is to say, in the both-face image formation mode wherein, after four color toner images are transferred onto a front surface (first surface) of the recording material (transfer material) and the toner images are fixed by the fixing device, the recording material is turned up (reversely rotated) to use a back surface (second surface) of the recording material as a front surface and is born on the transfer drum 5a in that condition to transfer the toner images onto the second surface, fixing oil adhered from a fixing roller of the fixing device 9 to the first surface of the recording material during the transferring of the toner images onto the first surface is transferred onto and adhered to the bearing sheet 5f of the transfer drum 5a when the toner images are transferred onto the second surface of the recording material born on the transfer drum. And, the fixing oil adhered to the bearing sheet 5f is transferred onto and adhered to the photosensitive drum 1 at the transfer station (where the bearing sheet 5f is contacted with the photosensitive drum 1) during the idle rotation of the transfer drum 5a (for cleaning) after the recording material on the second surface of which the toner images were transferred is separated from the transfer drum 5a.
As a result, when the both-face image formation are continuously effected with respect to a plurality of recording materials, the fixing oil is trapped on the photosensitive drum 1. Consequently, if the further image formation tries to continue, the toner from the developing means will be adhered to a portion on the photosensitive drum which should be kept white due to the viscosity of the fixing oil, thereby causing the fog in the transferred image. Further, during the developing operation, the toner adhered to a portion on the photosensitive drum which should be kept all black is difficult to be separated from the photosensitive drum due to the viscosity of the fixing oil, with the result that the toner is not transferred onto the recording material sufficiently, thereby causing the transferred image thinner. In this way, in the cleaning system wherein the toner is removed from the transfer drum after the recording material is separated from the transfer drum and then the transfer drum is idly rotated by one revolution, since the fixing oil adhered to the recording material during the both-face image formation is transferred onto the photosensitive drum during the idle rotation of the transfer drum to contaminate the photosensitive drum, the image is deteriorated. Thus, this system is not preferable.
Accordingly, in order to prevent the deterioration of the image due to the oil contamination of the photosensitive drum during the both-face image formation, it is necessary to clean the bearing sheet during the rotation of the transfer drum for separating the recording material from the transfer drum. However, as mentioned above, when the length L1 of the recording material is greater than the distance L2 between the transfer station and the cleaner, since the last color toner image is still being transferred onto the recording material, the color deviation of the last toner image cannot be avoided.